More particularly, the invention relates to a method which makes it possible to continue to have access to a set of services within a private communications network, said network enabling a plurality of items of equipment to communicate with one another via two-way communications links, even in the event of a dedicated link failing, thereby causing the communications network in question to be split.
The general context of the invention is that of a network of items of equipment made up in particular of nodes e.g. representing private exchanges interconnected by two-way communications trunks, said network being connected to a circuit-switched public network.
In general, a trunk between two terminals defines a set of accesses to the terminals. These accesses are transmission lines rented or purchased by users from a public service. Physically accesses comprise a plurality of channels. Thus, for example, a “basic” T0 type access carries two data transmission channels known as B channels operating at a bit rate of 64 kilobits per second (Kbps), plus one signaling channel known as a D channel whose data rate is 16 Kbps. So-called “primary” T2 type accesses support thirty B channels and one D channel, with all of the channels operating at a rate of 64 Kbps.
All of those accesses are said to be multipurpose in the sense that each of them is capable, within the limit of its own data rate, of supplying all of the services offered by the network. In general, a B channel is a channel that can be used for carrying any type of information, whether its frames contain data or voice. A D channel is a channel suitable for carrying signaling messages specific to various services. The D channel is used essentially for setting up a call over one of the channels within the access itself. By way of example it conveys information sent in the form of a set-up message by a terminal which is requesting a call with another terminal. This information can be a D channel number, a destination address, a type of service, . . . .
The various signaling or data channels are rented from a public service by the user. The signaling channel or D channel is set up permanently between the two terminals that are situated at its ends. The user thus pays for permanent use of this channel. However, the cost of the data channels or B channels depends on the type of access which is associated therewith. The cost of these channels can be made up of a flat rate subscription plus an amount that depends on call time and on call distance. The user of a network of the type to which the method of the invention applies therefore keeps down the number of B channels rented and keeps down the use that is made of them for reasons of economy. In addition, the D channels can be used to transmit some data. This data is not telephone data but rather packets of data, e.g. as passed between two computers or terminals. Since the cost of using the D channels does not depend on the volume of information passing over the channel, the user makes maximum use of the D channel.
Thus, the signaling channel can be used to convey digital signals enabling a variety of services to be made available within the network in question. For example, these services can be options made available to a user, such as automatically transferring calls from one terminal of the network to another terminal of the network, automatic recalls, intercepting calls, . . . .
Signaling channels are also heavily used in call distribution methods. The essential function of call distribution methods is to put an outside user seeking to benefit from a specific service or to obtain some particular information into communication with a particular operator having the appropriate competence for replying to the user's request. In call distribution facilities, all of the information relating to a dynamic routing travels over the signaling channels. Dynamic routing makes it possible instantaneously to analyze the state of the call distribution facility together with the resources that are available at each instant; it thus makes it possible to calculate the best possible path for the call to follow in order to reach an agent having appropriate competence as quickly as possible.
The use of signaling channels also makes it possible to implement virtual private networks (VPNs). In this type of network, numerous signaling channels are specific to the network and to its needs. The signaling signals which are transmitted over the signaling channel make it possible to provide numerous services satisfying demand from users of the network.
However, in VPNs, a public telecommunications network continues to be used for transmitting information other than signaling information. Thus, digital data packets, signals carrying voice, . . . , are transmitted over the public telecommunications network. Electronic correlation units then enable data traveling over the public telecommunications network to be recovered and made to correspond with the signaling signals that have been transmitted over the signaling channel.
Unfortunately, certain types of fault can occur and prevent signaling signals from transiting via the usual signaling channel. Such a fault can be associated with an access, with a trunk itself, or with an anomaly at one of the nodes of the network.
When a signaling channel which exists between two nodes of a network in normal operation is no longer usable, the principle of dynamic routing can enable a new path to be defined for the signaling signals. This new path enables the services to continue to be implemented over the entire network.
However, certain types of fault can lead to the network becoming split. A network is said to be “split” when the initial network is subdivided into two or more portions which can no longer communicate with one another. A private communications networks that has split then restricts the services that the network would otherwise offer in normal operation. For example, with a call distribution facility, certain operators can no longer be reached; their workload is consequently greatly reduced, while the workload on operators who can still be reached quickly becomes excessive. As a result, an outside user is put into communication with an operator more slowly, and even then the operator is perhaps less competent than the operator who would have been reached in normal operation. The outside user's opinion about the services on offer is then necessarily less good.
In order to resolve problems of a private network becoming split, the state of the art proposes so-called “redundancy” solutions which consist in taking the preventative measure of always doubling up existing signaling channels. Thus, for each signaling channel in use, there is also an identical signaling channel whose sole function is to take over the role of the first signaling channel in the event of that channel becoming faulty. However, as mentioned above, the cost of a signaling channel is not related to the volume of information that passes over the channel; merely renting it is expensive. Consequently, redundancy solutions are excessively expensive and are not necessarily satisfactory in the event where faulty operation of the network does not stem from a failed trunk but from a breakdown in one of the terminals.